Reduced impurity filament for electric lamps

ABSTRACT

An annealed tungsten filament wire for electric lamps having substantially the same amounts of impurities therein as was present prior to annealing.

[111 3,812,393 45] May 21, 1974 United States Patent [191 K00 et al.

[58] Field of 313/341, 344, 345

References Cited UNITED STATES PATENTS Shurgan, Washington Township.Bergen County, both of NJ. Assignee: Duro-Test Corporation, North3,341,917 Oyabu et al. 313/344 X 2,813,995 Watts............. 313/344 X3,346,761 Ackerman 313/344 X Bergen, NJ.

[22] Filed:

Feb. 4, 1972 Primary Examiner-Paul L. Gensler AppL Na: 223,607 Attorney,Agent, or Firm-Darby & Darby mum r w C mfm %f 10- e gs [BUM nnu uw .mmmmwaam t 0 mm R m O Sh .nD Bfi m 3 Antm w H a awmfiP tmsl d kmw mwa n n91 lamm 7nve a UAhm 58 m m B w mm A 3 5 4H 2 m mm 2 D W MM 2 m. M r u&.B" D. w U" f M m 9 "n m mm d o m M dwm U n CG .UO, MW m C1 U] M N 6 55.1 [.1

REDUCED IMPURITY FILAMENT FOR ELECTRIC LAMPS This application is acontinuation-in-part of our prior copending application Ser. No. 66,275filed Aug. 24, 1970, now US. Pat. 3,662,789, dated May 16, 1972 entitledMandrel for Manufacturing Filament Coils and Method for ManufacturingFilament Coils which is assigned to the same assignee.

BACKGROUND OF THE INVENTION Filaments of tungsten wire are in widespreaduse for incandescent and fluorescent lamps. In recognition of thedetrimental effects of impurities on the metallurgical properties oftungsten, lamp manufacturers take extraordinary steps to producetungsten wire having a minimum amount of residual impurities. Forexample, tungsten ore of the highest purity is used, and, to preventimpurity contamination during processing, the reduction of tungstenoxide is generally carried out in tungsten boats. Furthermore, as aroutine quality control procedure, impurity analyses are carried out onthe material at various stages of the powder-metallurgy process.

While a high degree of care is exercised in the manufacture of thefilament wire, the manufacture of the filaments themselves has remainedbasically unchanged for the past 40 years. The conventionalmanufacturing procedure, in general, does not take into account anyimpurities which are introduced into the filament nor the deleteriouseffects of these impurities.

Tungsten filaments for electric lamps are commonly manufactured in theform of coils or coiled coils, the latter comprising a coil which isitself coiled. The most commonly used method for making such filamentcoils comprises the following steps: (1) winding tungsten wire as a coilon an elongated mandrel; (2) annealing the coil while still on themandrel by passing it through a hydrogen furnace maintained at anelevated temperature, usually about 1,200C.; (3) cutting the mandrel andcoil to the desired length of the individual filaments; (4) dissolvingaway the individual mandrels in a suitable acid such as hydrochloricacid; and (5) reannealing the tungsten wire in wet hydrogen at anelevated temperature, usually around l,300C., for cleanmg.

The two materials for mandrels in common use throughout the lampindustry are steel and molybdenum. Aside from economics, the choice ofmandrel material is severely limited by a large number of technicalrequirements. The most significant of these requirements for the mandrelare: (1) high tensile strength is required for filament winding andannealing under tension without plastic deformation of the mandrel; (2)the melting point of the mandrel must be above the annealing temperaturerequired to set the filament coil prior to cutting; (3) the temperaturecoefficient of expansion of the mandrel should be close to that of thefilament coil at the annealing temperature; (4) an adequate amount ofbonding is needed between the filament coil and the mandrel duringannealing to assure the retention of coil geometry upon subsequentcutting of the mandrel into individual filaments; and (5) the mandrelmust be capable of being dissolved chemically without affecting thetungsten coil. For the foregoing reasons steel mandrels have been andare currently being used almost universally for most coiled filamentswhile molybdenum mandrels are used for coiled-coil filaments whichrequire annealing temperatures above the melting point of steel.

The use of steel for the mandrel material, although economical, isundesirable from the standpoint of quality of the filaments produced,since steel has an adverse effect on the metallurgical properties of thetungsten filament during the manufacturing process. The main reason isthat in forming the bond between the coil and the mandrel during theannealing step, a small amount of iron inevitably diffuses into andembrittles the tungsten. To understand this it should be considered thatin the heavily drawn tungsten wire used as lamp filaments, a fibroussubstructure exists prior to recrystallization, the average subgrainsize of this structure being less than one micron. It is well recognizedthat substitutional diffusion of elements, such as iron, in tungstenoccurs much more rapidly along sub-boundaries and grain boundaries ofthe tungsten than within the grains through the normal lattice sites.Since the activation energy for volume diffusion (within the grains)being around 120 Kcal/mole for iron in tungsten is several times higherthan that for interfacial diffusion (along boundaries), an appreciableamount of interfacial diffusion can occur readily at temperatures lessthan one-half that of the tungsten. Therefore for tungsten, which has ahigh concentration of sub-boundaries and grain boundaries per unitvolume when formed as heavily drawn wire used in filaments, asubstantial amount of iron diffuses into the tungsten preferentiallyalong the boundaries during annealing at a relatively low temperature.Furthermore, a concentration gradient of iron in the tungsten coil alsoexists, which decreases from the inner coil surface, in contact with themandrel, to the outer coil surface, which is never in contact with themandrel. 1

For tungsten filaments made by conventional methods on a steel mandrel,it has been substantiated, by impurity analyses on annealed coils madeafter dissolving away the steel mandrel, that an appreciable amount ofiron diffuses into the filament coil. The amount of iron present afterannealing varies from one coil segment to another, and depends upon theprior history of the tungsten wire. For tungsten wire approximately 2.5mils in diameter, annealing in the manner described above typicallyresults in an increase in iron concentration up to 50-100 ppm (byweight), as compared with 10 ppm or less of iron in the wire prior toannealing. Analyses of the surface material etched off from the annealedcoil shows concentrations of iron substantially above ppm.

The presence of localized segregations of iron diffused into a tungstenwire filament coil has been found to be responsible for the formation ofnumerous slivers of iron on annealed filament coils after the mandrelhas been dissolved away. When the coil is stretched out, the slivers areevident on the inner surface of the coil at the areas where it wasoriginally in contact with the steel mandrel. The amount of sliveringincreases with increasing concentration of iron, and is absent whereverthe diffusion of iron into the tungsten wire coil is prevented.

Tungsten wire filament coils having excessive segregation of iron arealso brittle and contribute to shrinkage (rejects) in the manufacture ofelectric lamps. Fracture of the filaments frequently occurs at the innerside of the coil being clamped by the leads during mounting of the coilin a lamp. This is indicative of the strong embrittlement effect oflocalized iron, since the compressive stresses at the inner side of thecoil should favor plastic flow instead of crack initiation.

Another detrimental effect of iron is to reduce the advantages achievedby doping in non-sag filaments. Incandescent lamp filaments are normallydoped with small quantities of aluminum, silicon, and potassiumcompounds to raise the recrystallization temperature and to develop aninterlocking grain structure characteristic of sag resistant tungsten atelevated temperatures. It is well known that iron diffused into dopedtungsten reduces the recrystallization temperature and develops anon-interlocking equi-axed grain structure, partially nullifying theeffect of dopants in producing a non-sag material.

In accordance with the present invention a new type mandrel is utilizedfor forming filament coils as well as a new method for manufacturingfilament coils. These provide a filament in which the amount ofimpurities introduced therein during manufacture can be controlled andreduced. Filaments made in accordance with the subject invention havebeen found to have greatly improved operating characteristics, due tothe reduction in the impurity content.

In accordance with a preferred embodiment of the invention, a materialwhich has a lower melting point than the filament material and does notalloy therewith is coated. over an inner core of the mandrel. The coilis then wound over the coated mandrel and is annealed. The annealed coilis cut into desired lengths and the mandrel is dissolved from the core.Upon annealing of the filament, the coating material forms a strong bondwith the coil and serves as a barrier to the diffusion of the inner corematerial into the filament coil. This produces a filament havingsubstantially no additional impurities diffused therein from what waspresent in the filament material prior to annealing. The coatingmaterial on the mandrel also eliminates the formation of slivers on andthe embrittlement of the annealed coil. Further, the coating materialalso aids in speeding the dissolving process.

In a preferred embodiment of the invention, copper or acopper alloy isused as the coating material over an inner mandrel core of steel. Thecopper or copper alloy melts during the annealing of the filament coiland forms a bond therewith to provide a better geometrical set. Themandrels also can be dissolved very rapidly from the coils using asuitable acid, such as nitric acid.

As another aspect of the invention, the coating material can be appliedto the wire rather than to the mandrel.

p It is therefore an object of the present invention to provideanimproved filament for use with electric lamps.

A further object is to provide an improved tungsten filament for anelectric lamp which, after annealing, has substantially the same amountof impurities present as before annealing.

Other objects and advantages of the present invention will become moreapparent upon reference to the following specification and annexeddrawings, in which:

FIG. 1 is a perspective view of the mandrel of the subject inventionshowing the filament coil wound thereon;

, FIG. 2 is a cross-section of an annealed coil. shown on a mandrel; and

FIG. 3 is a cross-sectional view of an annealed coil according toanother embodiment of the invention.

Referring to FIG. 1, a mandrel 10 has an inner core 12 on which iscoated a thin layer of a suitable material 14. In the preferredembodiment of the invention being described, the core 12 is steel andthe material of the coating 14 is copper. Alloys of copper also can beused as is described below. Where steel is used as the inner core andcopper or a copper alloy as the coating material, the latter can beplated or clad on to the inner core.

A coil 16 of filament wire is wound on the outer layer 14 out of directcontact with the mandrel inner core 12. The wire is, for example, oftungsten material and any suitable number of turns per inch of the coilcan be wound. Copper is used as the preferred coating material whenworking with a tungsten. The choice of copper in conjunction withtungsten wire is advantageous in that copper readily wets tungsten buthas no detectable solubility in tungsten.

Considering now the preferred embodiment of the manufacturing processfor the mandrel shown in FIG. 1, the copper or a copper alloy is coatedonto a steel wire core which is initially at a relatively large diameterwire size. For example, the wire is in the order of 0.1

inch in diameter and the thickness of the coating is. i

0.003 inch. The coated steel wire is then drawn to a fine wire size, forexample, in the order of 0.01 inch in diameter with the coating layerhaving a thickness of about 0.0003 inch. By doing this the copper orcopper alloy becomes sufficiently work-hardened to the extent that itresists deformation during winding of the coil on the mandrel. Thethickness of the coating needed depends in large measure upon themandrel diameter.

To produce the improved filament, in accordance with the preferredembodiment of the invention a coil of tungsten wire is wound on themandrel with the desired number of turns per inch. The tungsten coil onthe mandrel is then annealed at a temperature around l,l00C by passingit through a tube type furnace containing a non-oxidizing atmosphere ofhydrogen or nitrogen. The mandrel and coil are then cooled to roomtemperature.

After the annealing and cooling, the coil and the mandrel are cut todesired lengths. The individual cut mandrels are then dissolved from thecoils. To do the latter, the individual coils and mandrels are placed ina container which is partially filled with water at room temperature.Concentrated nitric acid is then added to the water to attain an acidconcentration of 25 to 35 percent or, preferably, the acid is mixed tothe desired concentration before the coils are placed in the container.The entire mandrel is dissolved rapidly in this solution leaving thecoil. The tungsten coils are ready to be used as filaments after theyare removed from the acid bath and washed.

The present invention has numerous advantages with respect to thecopper-coated mandrel itself. Copper has a melting point within thetemperature range required to set the tungsten coil by annealing. Visualinspection of the annealed coils made in accordance with the inventionshowed that the copper was molten during the annealing process (themelting point of copper is 1,083C.) and that a strong bond was formedbetween the coil and the mandrel copper layer upon cooling. This isshown in FIG. 2 where bonding points 18 of the coil to the previouslymolten coating material 14 are shown. The bond has been found to bestronger than that normally obtained from a coil wound on a steelmandrel without copper coating. The reason for this is that the flow ofmolten copper partially around the tungsten wire provides a larger areaof bonding between the tungsten wire and the copper coated mandrel. Thestrong bond formed between the filament coil and the copper upon meltingand resolidifying of the copper aids in retaining the coil geometry uponcutting the annealed continuous coil into the desired lengths ofindividual filaments.

As a further advantage, the surface tension of copper is sufficientlyhigh so that the molten copper does not drip off during annealing of thefilament and remains as a layer over the steel core, the latter neverbeing in contact with the tungsten coil. This is also illustrated inFIG. 2. The bond is formed by resolidification of the molten coppercoating 14 partially around the tungsten coil 16 at points 18 with alayer of copper 14 remaining over the inner steel core 12. It has beenfound that a copper coating of 0.0002 inch thick is sufficient toprevent the diffusion of any significant quantities of iron from thesteel core to other impurities into the tungsten filament coil forannealing times on the order of seconds at l,lO0C.

Another advantage of using copper-covered steel mandrel is the increasein efficiency of coil processing through the use of nitric acid indissolving the mandrel. In the conventional prior art method, steelmandrels are dissolved in hydrochloric acid and require as long as overone hour for complete dissolution of mandrels of large diameter. In thepresent invention, the coppercovered mandrels are dissolved rapidly innitric acid, preferably 25 percent nitric acid to which is added 10percent sulfuric acid. For example, complete dissolution of steelmandrels 0.011 inch diameter in hydrochloric acid requires about 25minutes, whereas copper-covered steel mandrels of the same size arecompletely dissolved in a nitric acid sulfuric acid bath within oneminute.

That the entire copper-covered mandrel can be dissolved rapidly in arelatively concentrated solution of nitric acid is attributed to thedepassivating effect of the copper coating on the inner steel core.Without the copper coating, steel becomes passivated in nitric acid atconcentrations above approximately 20 percent and resists attack by theacid. The copper-covered steel mandrels, however, are readily dissolvedin nitric acid at concentrations up to 50 percent.

It has been found that the use of the copper-covered steel mandrel andthe process for dissolvingthe mandrels further improves the quality ofthe completed coiled filaments. Since the copper-covered steel mandrelscan be dissolved within a few minutes without applying heat, the weightloss on coils which occurs during mandrel dissolving is substantiallyless than on coils processed with hydrochloric acid, typically theweight loss being reduced by 50 percent. The decrease in weight lossincreases the uniformity of processed coils and the lumen output of thefilament used in a lamp.

An additional advantage of dissolving the coppercovered steel mandrel innitric acid is that the aquadag coating on the tungsten coil issimultaneously removed. In the conventional method of dissolving theuncoated steel mandrel in hydrochloric acid, the aquadag remains on thetungsten-coil and is removed by subsequent firing of the coils in wet.hydrogen above l,400C. In the present invention, firing the coils forcleaning is often times no longer necessary. Coils from which themandrels are dissolved away in nitric acid at temperatures above C aregenerally free of Aquadag and can be used in lamps without furthercleaning. This contributes to a reduction in manufacturing cost.

The present invention is not restricted to pure copper as the coatingmaterial and a specific type of steel as the inner core. Alloys ofcopper also can be used for the coating material. For example,copper-base alloys coated on the steel mandrel can achieve the sameresults as are described above. The addition of l 5 percent of silver orindium into copper decreases the melting point of pure copper. Sincealloys of these compositions exhibit a liquid-solid region extendingthrough a wide temperature range, the alloys remain partially molten ina temperature range lower than the melting point 'of pure copper. Thishas two advantages over pure copper. First, control of the annealingtemperature of the tungsten coil is less critical. Second, the abilityto form a bond between the coil and the mandrel at lower temperaturesfurther improves the ductility of the tungsten since the inherentductility of heavily drawn tungsten wire decreases with increasingannealing temperature. The presence of these alloying elements, such asindium or silver, has no detrimental effect on the tungsten coil becauseof the lack of solubility of these elements in tungsten.

Filament coils processed in the manner previously described oncopper-covered or copper-alloyed mandrels have been inspected and havebeen found to be substantially completely free of slivers. Thisindicates the absence of diffusion of iron into the coil. The absence ofslivering is also accompanied by a marked increase in ductility. Theincrease in ductility is most pronounced on coils which are fullyrecrystallized; that is,

annealed or flashed at temperatures above 2,000C. Recrystallized coilsprocessed on copper-covered steel mandrels can be stretched at roomtemperature for length several times greater than coils processedsimilarly on bare steel mandrels before fracture occurs. This not onlyimproves the shrinkage, decreases waste in coil mounting duringmanufacturing, but also increase the cold-shock resistance of filamentswhen mounted in lamps.

Filaments made in accordance with subject invention have been found tohave substantially the same amounts of impurities after annealing as waspresent in the filament material prior to annealing. This is believeddue to the fact that the coating material on the mandrel acts as abarrier against diffusion of the mandrel core material into thefilament. Tungsten filament wire made in accordance with prior arttechniques on a steel mandrel has an increase in iron concentration upto 50-100 ppm (by weight) as compared with 10 ppm or less of iron in thewire prior to annealing. Tungsten filament wire made in accordance withsubject invention has substantially the same amount of iron and otherimpurities present after annealing as was present before annealing. Ingeneral, the iron concentration is normally less than 15 ppm.

FIG. 3 shows another embodiment of the invention. Here, the mandrel core12, which is illustratively of steel is left uncoated. The core 12 isformed to the decoated wire, the wire 16 itself is first formed to thedesired diameter, for example by drawing. The coating 21 is then appliedto the desired thickness. One suitable process for applying the coatingis by electroplating. Other conventional processes also can be used.

The manufacture of filaments using the coated wire and the uncoatedmandrel, of FIG. 3 is carried out in the same manner as previouslydescribed after the coated wire is wound over the mandrel. F IG. 3 showsthe wire over the mandrel at a time after the annealing has been carriedout. As seen, an amount of the coating material 21 has melted duringannealing from the wire to form the fillet shaped areas 22. However, anamount of coating material 21 remains between the mandrel core 12 andthe wire 16 to prevent the impurities from entering the wire. Themandrel and coating are dissolved and the processing of the filament iscompleted in the manner previously described.

In both of the embodiments of the invention described, it should beunderstood that the coating, of either the filament wire or the mandrel,aids in preventing impurities from the mandrel and other sources fromentering the filament wire.

The present invention also finds application in the manufacture ofcoiled-coil filaments. Here, the molybdenum mandrel is coated or thewire is coated. The acids used to dissolve the mandrel are selectedaccordingly.

Tungsten filaments made in accordance with the subject invention alsoretain the high tensile strength and ductility characteristic ofuncontaminated, unrecrystallized, heavily drawn tungsten wire. Incontrast, tungsten filaments made on uncoated steel mandrels becomebrittle and weak, the extent of embrittlement and weakening being afunction of the amount of iron diffused into the tungsten wire coilduring annealing. As a typical example, C-9 tungsten filaments made inaccordance with the invention for a 60 watt lamp, have a uniformly highbreaking load in tension in the range of 406-418 grams. Tungstenfilaments processed on an uncoated steel mandrel exhibit breaking loadsover a wide range, as low as 31 grams.

What is claimed is:

1. A filament for an electric lamp of the type coiled and annealed on awire mandrel containing iron, said filament consisting of a coil ofannealed tungsten wire having an uncoated surface and less than 15 partsper million (by weight) of iron impurities.

